it is coming from nbc pete williams. a senior law enforcement official says it appears there were two unbloweded devices. just two that went off. police are studying surveillance to see if they can see anybody with packages where the bombs went off. there is a video of someone carrying two backpacks but it is too early to know whether that was related to the attack. >> social media played a huge role in the last 24 hours. the aftermath of the bombings at the boston marathon. boston police confirmed the explosions in a tweet at 3:39 p.m. eastern time and provided updates and subsequent tweets throughout the afternoon and into the evening. i'm not sure this had been done before. witnesses sent updates as people around the country followed the attack. and foursquare was actually running in the marathon, the ceo. i'm okay. about 20k of us in coral just before mile 26 marker. the hashtag spiked after the attack. and google set up what's called a person finder page for the marathon sphoeexplosions. it was used in a couple othe